The present invention relates to Vertical Cavity Surface Emitting Laser (VCSEL) chips and subassemblies.
VCSELs are an important optical source for fiber optic data communication systems. Most of the devices that have been used in these systems emit light in the 830 to 860 nm wavelength range. However, VCSELs have been fabricated that have demonstrated emission at wavelengths equal to or in the vicinity of the following wavelength values: 660 nm, 780 nm, 850 nm, 980 nm, 1310 nm, and 1550 nm.
Red light emission VCSELs (˜660 nm) are of considerable interest for applications in which the emission light capable of being seen by the human eye is valued. For instance, photoelectric sensors attuned to detecting such emission light might be used to sense the presence/absence, distance or other attribute of objects illuminated by that light. The ability of an observer to see the emitted beam for sensor alignment purposes is advantageous. A bar code scanner would be a special case of such a sensor, and users prefer the use light thereby of a visible wavelength so that they can more easily aim the light beam at the bar code. Chemical, biological, or medical sensors can take advantage of the absorption or scattering of light having a particular emitted wavelength or wavelength range. An example of this would be a pulse oximeter which relies on the relative absorption of 665 nm and 905 nm wavelength light sources to determine the oxygen content of blood being measured. Display or printing devices may rely on emitted light of such shorter wavelengths to provide higher resolution.
The intended use or uses for systems with such red light emission VCSELs in them determines the attributes thereof that are of interest including emitted light wavelength, power conversion efficiency, emission divergence angle and the emission mode structure. The mode structure describes the shape of the emitted beam. Some uses require a single mode device, i.e. a device with a uniform round Guassian shaped light emission intensity profile.
FIG. 1 is a fairly general schematic layer diagram of a typical thin-film semiconductor material red light emitting VCSEL structure formed on a substrate, 1. The mirrors, 2 and 3, forming the optical resonance cavity are constructed from AlGaAs materials having relatively large refractive index thin-film layers with a composition of approximately Al0.5Gs0.5As alternating with relatively small refractive index thin-film layers with a composition of AlxGa1-xAs where the mole fraction x>0.85. Each such layer has a thickness corresponding to one quarter of an optical wavelength (λ/4) for the light intended to be emitted by the VCSEL in the material of interest for that layer. The optical thickness is defined by the wavelength divided by the refractive index. For instance, if the emission wavelength is 670 nm, and the composition is GaInP which has a refractive index of 3.65. The optical thickness corresponding to one wavelength in the material would then be (670 nm)/3.65=183.6 nm. Within the mirror, the layers are one quarter wavelength thick, and so the mirror layers would be in the range of 45 nm thick. Many periods (>20) of alternating quarter wavelength thick layers of these two materials forms a highly reflective mirror at the intended emission wavelength. Mirror 2 is doped to be of n-type conductivity, and mirror 3 is doped to be of p-type conductivity with a highly doped doping grading layer, 3′, thereon having a thickness of 2nλ/4 with n being an integer.
The active region, 4, of the VCSEL between mirrors 2 and 3 is formed from a AlGaInP materials system. One or more quantum wells are included in the structure formed of corresponding thin-films with a composition approximately equal to Ga0.5In0.5P. The injected carriers are captured by these quantum wells and then combine to thereby emit light. The composition and thickness of each quantum well thin-film together determines the emission, or photoluminescence, wavelength of the quantum wells. The quantum wells are spaced apart by barrier thin-film layers of AlGaInP, and together are bounded on either side in active region 4 by cladding, or confining, layers, 5 and 6, also of AlGaInP, with the compositions both barrier and cladding layers being chosen such that they are lattice constant matched to GaAs which will serve as the device substrate, and so that they have a bandgap that is larger than that of GaInP for thereby providing photon confinement. The total thickness of the active region is typically one wavelength (1λ) of the light intended to be emitted by the VCSEL thick although it can be any integer multiple of one half of the emission wavelength (nλ/2). A highly doped GaAs capping layer, 7, is provided on doping grading layer 3′ to together reduce electrical resistance in lateral directions.
One of the constraints for the overall VCSEL epitaxial structure is that the lattice constant or parameter of the layers in the structure be nearly equal to that of the underlying GaAs substrate. If this is not true, then lattice defects can form which may cause damage to the device as the device is used and so limit the reliability or lifetime of the device. In the AlGaAs materials system, this condition is met for all possible compositions trading off aluminum for gallium ranging from AlAs to GaAs. However, in the AlGaInP materials system, this condition is met only for the compositions corresponding to (AlxGa1-x)yIn1-yP, where the mole fraction y=0.51. The mole fraction x can be adjusted from 0 to 1.0 without affecting the lattice match to GaAs. However, the bandgap discontinuities in AlGaInP can be adjusted somewhat by adding small amounts of strain to the quantum wells and barrier layers by slightly adjusting the value of y and the thickness of the layers. If the total thickness of the strained layers is kept sufficiently thin (100-200 nm) then defects do not form, and the device reliability is not affected.
Confinement of electrical currents to desired locations in the structure can be provided with the standard techniques of ion implantation and oxide aperture formation as shown in the more general schematic layer diagram of FIG. 2 of the FIG. 1 VCSEL structure having the same semiconductor material layers. There, such structures as an implant or oxide confining layer, 8, and a top metal interconnection, 9, with an emission aperture are indicated. Substrate 1 has another metal interconnection, 1′, provided on the exposed outer surface thereof. Other useable alternatives exist for this purpose which have been demonstrated in VCSELs emitting light at other wavelengths.
Red light emission VCSELs have been demonstrated, but, typically, the temperature range of operation is limited and the maximum output power, particularly single mode output power, is also limited. These limits become more significant for shorter wavelength devices. Due to the small bandgap discontinuities and the low thermal conductivities in AlGaInP—AlGaAs material systems, the output power of red light emission VCSELs decreases if the emission wavelength decreases or the operation temperature increases, or both. Small bandgap discontinuities means that carriers that should be captured in the quantum wells and recombined there to emit light, instead, escape and so don't contribute to the light output. As the temperature increases, the charge carriers are even more likely to escape those wells. For shorter wavelength devices, the quantum well needs to be shallower in order to generate the higher energy, or shorter wavelength, light but this also contributes to the escape of charge carriers. Thus, there is a very significant increase in difficulty between achieving high performance in a device emitting at 650 nm versus one emitting at 670 nm, for instance.
Another issue which plays a role in the temperature range of operation is the electrical resistance of the AlGaAs mirrors. The Al0.5Ga0.5As composition, which constitutes approximately 50% of the mirror thickness, has poor thermal conductivity. In addition, the many periods in the mirror contributes to an increase in the resistance, which results in additional heating. The additional heating combined with the extra sensitivity of this material system to temperature only exacerbates the problem. These factors combine to make providing a red light emission VCSEL device with substantial single mode output power particularly difficult. Further, the smaller aperture size of the single mode device typically means that these devices heat more quickly. Thus, there is a desire to have a red light emission VCSEL device configured to be less bounded by such limitations.